With respect to glare estimation of outdoor sports facilities such as athletic fields, sports stadiums, etc., for example, JIS 29110 (JIS: Japanese Industrial Standards) (which corresponds to CIE112-1994 (CIE: Commission Internationale de l'Eclairage)) adopts, as an index value, a glare rating which represents a glare level with a numerical value between 0 and 100, and standard values of the glare rating are set for these outdoor sports facilities. The standard value of the glare rating is set by defining the maximum value of glare ratings in all horizontal directions around an observation point at the height of a player's visual line in the outdoor sports facilities so that glare does not become an obstacle to players. Accordingly, when glare estimation is performed, a worker repetitively measures glare gratings while changing his/her visual-line direction (measurement direction) within a horizontal plane around the observation point, and compares the maximum value of the measurement values of the glare ratings with the standard value to perform the glare estimation.
The measurement of glare ratings is generally performed by mounting a lens having a special optical characteristic called as a glare lens to a luminance meter, measuring equivalent veiling luminance and calculating the glare ratings on the basis of the equivalent veiling luminance according to a predetermined calculation. However, a glare lens having high precision is very expensive, and thus there has been recently proposed a technique of determining equivalent veiling luminance through image processing on the basis of a luminance distribution of an image picked up by an imaging camera (see JP-A-2004-087215, JP-A-2008-241380, for example).